Higher operating temperatures of gas turbine engines are continuously sought in order to increase the efficiency of such engines. As operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. A common solution is to thermally insulate certain components of a gas turbine engine in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed directly on the surface of a component have found wide use. Such coatings generally entail the deposition of a metallic bond layer onto the surface of a component, followed by a ceramic layer which serves to thermally insulate the component. Preferably, the metallic bond layer is formed from an oxidation-resistant alloy in order to promote the adhesion of the ceramic layer to the component. Various ceramic materials have been employed as the ceramic layer, particularly zirconia (ZrO.sub.2) stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides. These materials can be readily deposited by plasma spray techniques.
A primary objective of thermal barrier coating systems has been the formation of a more adherent ceramic layer which is less susceptible to spalling when subjected to thermal cycling. For this purpose, the prior art has proposed various types of coating systems, including the formation of ceramic layers having enhanced strain tolerance as a result of the presence of porosity, microcracks and segmentation of the ceramic layer. Microcracks generally denote random internal discontinuities within the ceramic layer, while segmentation indicates the presence of microcracks that extend through the thickness of the ceramic layer, thereby imparting a columnar structure to the ceramic layer.
As taught by Sumner et al. in an article entitled "Development of Improved-Durability Plasma Sprayed Ceramic Coatings for Gas Turbine Engines", published by the AIAA/SAE/ASME 16th Joint Propulsion Conference, Jun. 30 through Jul. 2, 1980, and Duvall et al. in an article entitled "Ceramic Thermal Barrier Coatings for Turbine Engine Components" ASME paper 82-GT-322, the presence of microcracks or segmentation can effectively serve as a strain relief mechanism for a thermal barrier coating, as evidenced by the results of controlled thermal cyclic testing.
Similarly, U.S. Pat. No. 5,073,433 to Taylor reports that the presence of vertical macrocracks homogeneously dispersed in a thermal barrier coating are capable of improving the thermal fatigue resistance of the coating. While Taylor employs the term "vertical macrocracks", Taylor's description depicts segmentation essentially identical to that of Sumner et al. and Duvall et al., with approximately 75 to 100 microcracks being present per linear inch of coating and each microcrack having a width of preferably less than 1/2 mil (about 13 micrometers).
A shortcoming of the above prior art is that the advantageous results obtained are generally restricted to strains thermally induced between the ceramic layer and the underlying substrate provided by the component. More specifically, the microcracks and segmentation provide strain relaxation for only local stresses, such as those generated by a mismatch in coefficients of thermal expansion of the ceramic layer and the substrate metal. As such, the microcracks are unable to provide adequate stress relaxation if the ceramic layer is relatively thick, such as on the order of about 0.75 millimeter or more, with correspondingly higher residual stresses.
The microcracks are also generally inadequate to provide protection from strains induced by a combination of thermal, mechanical and dynamic stresses, such as extremely high temperature gradients, compressive, tensile and hoop stresses, and vibrationally-induced stresses. When exposed to such conditions, large areas of the thermal barrier coating tend to spall or otherwise delaminate from the bond layer, because the microcracks do not provide sufficiently large discontinuities to arrest the propagation of cracks within the ceramic layer.
In addition, the methods by which the microcracks and segmentation are produced do not permit the location, spacing and size of the microcracks to be selectively controlled. As such, the thermal fatigue properties of components equipped with these thermal barrier coatings cannot be tailored to the specific geometry of a component or differences in operating environments at various locations on a component.
While the use of thermal barriers formed of ceramic tiles tends to solve some of the above shortcomings, their use is generally limited to relatively large articles whose applications warrant a relatively labor-intensive assembly procedure. However, weight consequences and adhesion is often a concern with ceramic tiles. Furthermore, ceramic tiles are unsuitable for protecting articles with complicated geometries.
Accordingly, it would be desirable to provide a thermal barrier coating characterized by the ability to resist spallation when subjected to hostile environments, particularly those in which a combination of thermal, mechanical and dynamic stresses are imposed. Preferably, such a coating would be capable of being tailored to specific applications in which the operating environment at various locations on an article may differ significantly, and in which the geometry of the article precludes the use of tiles.